Circuit Breaker Question

[Jeebusmn]

Hi,

I would appreciate any input that anyone has on my question. At the company I work at we have a piece of a equipement that draws 300-400 amps from a 3 phase 480 volt input. The piece of equipment is used to test regulators that are produced on-site.

Here is my dilemma. There is a 480 volt circuit breaker (not a building breaker) that was being used to turn on a piece of regulator test equipment. Occasionally, from what I am told the resistors and sometimes the circuit breakers would spark or explode. This made me very nervous since none of the energizing circuitry is enclosed. I told my superior about this and he got a few other people from a couple different departments involved to determine if this was safe. It turned out that no standard written company procedure was attached to using this test equipment. Somewhere someone dropped the ball. One of the older engineers then told us we should be using the house breaker to energize and deenergize the circuit so that we don't have to be close to the energizing circuit when we turn it on and hence not near by it if it explodes.

One of my fellow employees would like to build an enclosed case around the energizing circuitry as well as setting it up so we are flipping a 24 volt switch to turn on the test equipment instead of using the building breaker.

So, my question basically stems out of fear as to what happens when the building breaker fails. Is it safe to be using the building breaker to turn on and off a piece of equipment that draws quite a bit of current? Do building breakers fail from use? How much protection is lost if any by using the building breaker as an on-off switch? Is there anything that I can use in the NEC or elsewhere to support my case if this indeed is an unsafe way of turning something on and off?

This is my first real job out of college. I have a degree in electrical engineering and this is my first real job out of school. This whole ordeal has introduced me to office politics 101. I am interested in what anybody has to say (safe or unsafe) and if they know of anything I can use to back up my case because I am kind of green and no one will listen to what I have to say unless it is solidly supported.

Thanks.

Doug Millner

 

[tom baker]

Re: Circuit Breaker Question

Well, this is an interesting post. I appreciate your honesty and taking the time to post the drawing.

A case like this involves looking at the parts of your drawing,
Are they listed?
Used for its intended purpose?

How old
How well maintained

Some CB's are designed for switching, example HID rated breakers used for lighting.

Yes the building breaker could fail, the contacts could weld, especially if its old.

Perhaps a fused disconnect with time delay fuses could be added downstream of the bulding breaker.
The fuses could be sized to 125% of the normal load.
The fused disconnect has a very positive disconnect or opening of the circuit.

 

[don_resqcapt19]

Re: Circuit Breaker Question

It sounds like you need a short circuit current study. The available current fault current may exceed the ratings of the breakers and that would be one reason why it would explode. This can be very dangerous. You need to make sure all of the equipment is rated for the available fault current. In addition to having properly rated breakers, I would recommend the use of a remote controlled contactor.
Don

 

[augie47]

Re: Circuit Breaker Question

It's good that you question this. Basically I think Don has given you all the correct answers.
The "exploding breaker" is probably a result of being subjected to a higher fault current than it's rating. The remotely controlled contactor is a good idea, but they too have withstand ratings. You don't want the contactor to explode or weld shut. I also would recommend the remote controlled contactor protected by some properly selected current limiting fuses. You may learn another lesson about "politics" when you price the contactor. If you can't get idea to fly, I would definitely recommend current limiting fuses in a disconnect.

 

[Paul B]

Re: Circuit Breaker Question

Are you saying the line side draws 400 amp or are you showing 400 amp at the 35 volt DC?

 

[templdl]

Re: Circuit Breaker Question

Jeebusmn'
Breakers as an ON/OFF switch? They definitely can be. But unlike contactors which are intended for many 1000s of operations they do have a definite life. But unlike contactors breakers can clear faults.
"IF" the breaker is applied within its rating the UL489 endurance test reuires that a 400a breaker should provide at least 1000 full load operations and 4000 no load operations at 6 operation per minute.
There's a whole bunch of other testing that goes along with this that a commonly applied breaker normally isn't exposed to so one would would?expect a longer life. These tests include calibration checks at different points in the testing, overload tests, temperature rise checks, short circuit tests, interrupting tests, test that go way beyond the normal application requirements.
As far as the arcing is concerned, assuming that you have a molded case circuit breaker, if one examines the top of the breaker, the end where the contacts are located (some may refer to it as the line end) there is an opening on most breakers that provide a way for the arc gas to escape. So if a breaker is required to interrupt a fault there is a good chance that ionized gas may be expelled out the top. But under normal load interruption it is highly doubtful that this would occur.
In my 18 years working for a major breaker manufacturer and 5 years as an application engineer for molded case circuit breakers up to 600v 2500a, I have never ran across a failure unless the breaker was misapplied by installing it in as application which the breaker wasn't rated for.
I personalty see not problem with selecting a breaker rated for the application enclosed in a suitable listed enclosure and even put a 24vdc shunt trip in the breaker so that the breaker could be tripped remotely.
The only time I have seen welded contacts as being a problem is when a breaker is being teased magnetically where there is not enough overcurrent to trip the breaker thermally and not enough current to trip the breaker magnetically where the contacts may float, arc, puddle the contact material, the contacts never open and are kept closed and now the contacts are welded together. This is very rare.
Dave